Radiation from a Loop

Whatever you feed into the loop will be radiated. Its purely a passive device.

A) Get a pile of cash and buy a spectrum analyzer and antennas B) Take it to an EMI lab, along with a pile of cash

The FCC requires you to transmit your allowed power, not to emit freq or harmonics out of band or wider than each segment and not disturbed other equipment that is properly operating.

SWR (sanding wave ratio) is most likely what will get you in trouble.

This is the amount of power being reflected back to your transmitter verses what is going out. This also causes undesired frequencies to be generated. You can also get this effect bouncing off structures where it may not be apparent at the transmission sight. A field analyzer is more suited for those situations.

If the wavelenth is much (say, 10 times) longer then the perimeter of the loop, this is a trivial electromagnetic exercise. You got to know the area of the loop S, the current I and the frequency F.

E = IS Omega Mu_0 / 2 PI R^3

If the wavelength is comparable with the perimeter of the loop, then all depends on the geometry.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

Yes., bring it to me and i'll use my cheap MFJ (mighty fine junk) analyzer and leave the pile of cash when you go.. :)

Interesting... :)

Thanks for the start. I'll Google for more detail..

D from BC British Columbia Canada.

Yeah... It radiates. Either this is a loop antenna or a really big single turn inductor. Dunno how to look at it..

I was looking for a max emission strength equation as a function of I,V,f,X,Y etc. The loop is big like 2ft x 5ft.

D from BC British Columbia Canada.

I'd like to first try to make an FCC passable design on paper. That way I don't have to bring a pile of cash 3 or 4 more times.

D from BC British Columbia Canada.

or go buy a copy of "controlling radiated emissions by design", m. mardiguan. or Ott's "noise reduction techniques in electronic systems"

Cheers Terry

Great :( So.. Even if I create a simple emission model, the loop can exceed FCC limits by being in a room that creates standing waves?

Anybody here use a radiated EMI simulator?

D from BC British Columbia Canada.

Cont. Rad. Emissions by Design. Chapter 2 page 16. Ha! I got lucky! It's not an omitted page on Google books. :) By the way, this is looking like a good book.

Thanks!

D from BC British Columbia Canada.

IMO it is. very useful. which is why I wont lend my copy to anyone (its my 2nd copy - grrr)

Cheers Terry

Oh, if that's your only problem, just make A(Loop)=0. ;-) Other than one of the above, I don't thing the problem is solvable, at least without a lot more information. If it were that simple none of us would worry about EMI tests.

with the formulae in Ott et al, you can get an idea of the likely field strength. the result falls into one of 3 categories:

- way below threshold - probably OK

- way above threshold - probably bad

- somewhere inbetween - better look harder

of course in practice one does what you suggested, and forces A ~ 0 by design (where possible). IME I generally dont calculate anything, just implement best practice then do the tests. I have done the calcs a few times, including at the design stage (to choose between bipolars and BJTs in a particular app where I had to have adjacent traces running

Cheers Terry

That calculation would give you something like the field strength in Volts per metre in a given direction. Not that my maths are any good. ;-)

Does it really matter ? If the loop is small compared to wavelength then the efficiency is going to be small. If its resonant then its going to be a filter, abit a poor one. The converse is true.

If you put a dirty or clean signal into it it will still radiate that signal. From your point of view its the equipment connected to it that needs to be EMI clean and that includes power levels according to the frequency band specifications.

You have no control over whether your radiated signal is going to interact with something else to produce EMI ie rusty nail effect. To do that needs a fairly large signal.

well you need the current and voltage and the phase relationship etween them, so what you really want is the POWER going into the loop. If you can measure or determine the REAL power going in to the loop and the loop is a decent conductor then you can figure that all that POWER is radiated. Then it is easy to calculate the field strength at a distance.

Mark

nonsense!

Get some antenna modeling software. You might be able to do a quickee analysis by hand (dig up the AARL Antenna Handbook).

I ran across these sites recently. There might be something you can use here:

Wow! An old post of mine comes back!

D from BC British Columbia Canada